This invention is directed towards the field of color signature sensors used in process automation. More specifically, the invention is a method and an apparatus which performs color recognition of objects for the purposes of identification, sorting or matching.
Colorimeters are well known devices used for characterizing the color of an object and comparing the color of an object to the color of other objects. A lamp, either in the colorimeter or an external source, provides illumination which is reflected or transmitted by the object to a device which disperses light into an array of wavelength components. A detector array then converts the array of wavelength components into discrete signals which are representative of a color signature of the object. The discrete signals are then sent to an analog to digital converter and then on to a processor for processing. Processing involves a component by component subtraction of sensed component values from stored component values to produce a relative signature difference. Generally, as long as the relative signature difference falls within predetermined limits, the color of the sensed object will be acceptable.
This method of color differentiation is very accurate as long as the standoff distance, view angle and ambient lighting are held constant. However, in most industrial applications, it is not feasible to tightly control the presentation of the object to the colorimeter or the lighting of the object.
Another problem associated with colorimeters is that as the illuminating lamp ages, the color and intensity of the light produced by the lamp can change. This, in turn, causes the color signature of a sensed object to vary with the age of the lamp. This variation in color signature may result in many acceptable pieces being discarded due to erroneous color sensing.
One attempted solution to the problem of lamp aging is to increase the lamp voltage as the lamp ages in order to maintain a constant intensity and color output. However, it has been shown that changing the lamp voltage significantly affects lamp life. EQU (life.sub.actual /life.sub.design)=V.sub.design /V.sub.actual).sup.10.about.14
This means that a voltage increase of five percent will result in an approximate reduction in life of fifty percent.
Another possible solution to the problem of lamp aging is frequent recalibration of the colorimeter. However, this is only feasible in a laboratory environment. In commercial environments, access to the colorimeter may be difficult. Further, frequent recalibration increases the costs of scanning.
Thus, it is an object of the present invention to provide a colorimeter which compensates for changes in lighting, view angle and standoff distance. It is a further object of the present invention to provide colorimetric sensing which compensates for lamp aging without adversely affecting lamp life. It is yet another object of the present invention to provide colorimetric sensing which compensates for lamp aging without requiring frequent recalibration.